Just in case this helps anyone like me (not a pro), I just installed Sequoia 15.4.1 on my old iMac (late 2015) and it's great. It was a bit glitchy to install for me, but totally worth it to have a nicely running, current version OS. Was previously Monterey from memory - anyway a 4 x OS leap! And runs much better. Mainly the root patching had to be done a few times before all the updates were in place. I hate old computers needlessly getting thrown out and I love my 17-inch screen iMac, so I am one happy camper! It's using OpenCore Legacy Patcher from Github. I made my boot drive from my M1 laptop (as much faster) using a 32 GB USB and then selected the model of my iMac for the build. Details here to make it easy: https://www.youtube.com/watch?v=in5-3EjKFqA&t=506s